This invention relates to a discharge lamp operating circuit featuring decreased power loss of a switching device.
As described in JP-A-63-81795, the conventional discharge lamp operating circuit is constructed such that a capacitor and a discharge lamp are connected between the middle points of two series circuits connected in parallel with the power supply, each series circuit including two switching devices, and the discharge lamp is lit by turning on and off the switching devices alternately.
In the above-mentioned prior art, since a high voltage applied to set into operation the discharge lamp is applied also to the switching devices, the switching devices need to have a breakdown voltage higher than the high starting voltage mentioned above. The waveform of the current flowing through the switching devices and that of the voltage applied to the switching devices come to be close to rectangular, resulting in great power dissipation and increased cost unless MOSFETs are used which have high switching speed. Conversely, if the switching speed is increased, noise will become large, creating the possibility of noise interfering with the operation of other devices. With the prior art, the output voltage is about twice the supply voltage at the largest, and the prior art cannot be used in cases where the supply voltage is low.